In recent years, high critical current oxide superconducting materials are under diligent investigation for practical implementation. Such oxide superconductors hold great promise for application to long electrical wires, nuclear fusion reactors, maglev trains, accelerators, MRI tools, SMES devices, microwave filters, etc. In some fields of technology, practical applications have already been made until today. The oxide superconductors typically include bismuth-based, yttrium-based and thallium-based ones. Especially, the yttrium-based superconductor attracts the attention of researchers and engineers because this material offers the highest possible properties in magnetic fields at the liquid nitrogen temperature and, therefore, is the only one that is employable for linear motor cars with the aid of a liquid nitrogen cooling device.
This yttrium-based (Y-based) superconductor has the so-called perovskite structure, which is represented by a composition formula of YBa2Cu3O7-x. It is known that those materials with the yttrium (Y) of YBa2Cu3O7-x being replaced by a lanthanoid-based rare earth element and mixtures of such materials also exhibit superconductivity. Known examples of a fabrication method of these superconductor materials are a pulse laser deposition (PLD) method, liquid-phase epitaxy (LPE) method, electron beam (EB) method and metal organic deposition (MOD) method.
Superconductor fabrication methodology is typically categorized in two major approaches: an in-situ process and ex-situ process. The in-situ process is the method that performs both deposition of a metal required for making a superconductor and formation of a superconductor through oxidation at a time. The ex-situ process is the one that performs deposition of a material which is the base or starting material of a superconductor and thermal processing for forming the superconductor in a way independent of each other. Consequently, only in the ex-situ process, a precursor exists which is a pre-stage material prior to firing process. In cases where such precursor is obtained by firing, this is called the calcined or “precursor” film.
It is the in-situ process that has attracted attention in early stage of the development of superconductors because this process was thought to be a promising superconductor fabrication technique in light of its expected advantages: less process step number, and cost reducibility. However, later studies have revealed the fact that the in-situ process suffers from difficulties in obtaining excellent superconductors as it strictly requires that all of the film fabrication conditions must be set up together at a time. The ex-situ process, on the other hand, was first considered to have the risk of manufacturing cost increase. However, after development of a non-vacuum process, such as the metal-organic deposition (MOD) method or trifluoroacetate-metalorganic deposition (TFA-MOD) method, it became possible to attain noticeable manufacturing cost reduction. In addition, as thermal processing is readily controllable by use of a process with the heat treatments being performed at two separate steps, high properties are efficiently obtainable with excellent reproducibility. Accordingly, the ex-situ process now becomes a major process for fabricating yttrium (Y)-based superconductors.
Currently known ex-situ processes include EB, MOD and TFA-MOD methods. The EB method is a process having the steps of using an electron beam to deposit a precursor made of a chosen metal or else in a vacuum, and then applying thereto thermal processing or “firing” to thereby form a Y-based superconductor. Due to the presence of fluorine during firing process, it is predicted that crystal growth is performed while forming a quasi-liquid network as in TFA-MOD method. However, as this technique uses no carbon, any residual carbons do not exist in the resultant superconductor at all.
The MOD method has been long studied in other technical fields. Considerable efforts have been focused on the quest for reducing the amount of residual carbon that is harmful to Y-based superconductors. However, an effective residual carbon reduction technique is not found yet. To reduce the residual carbon, a need is felt to use a large-size electric furnace with good heat uniformity. The precursor of this technique is also called the precursor film in view of the fact that tentative or calcining process is done. This precursor is characterized in that it does not contain fluorine at all.
Regarding the TFA-MOD method, this is a derivative of the above-stated MOD method and has various features unique thereto. The TFA-MOD method is a technique using an organic matter, one feature of which lies in the use of a fluoride composition to thereby have a special mechanism capable of removing and excluding the carbon harmful to superconductor at the time of calcining process prior to firing process. Owing to this feature, the intended superconductor with enhanced properties is readily obtainable.
During firing process, the fluorine behaves to form a quasi-liquid phase network, which ensures that an atom-level oriented organization is formed by equilibrium chemical reaction with improved repeatability. Furthermore, the TFA-MOD method is a low-cost process that uses no vacuum during the steps of film fabrication, calcining process and firing process; so, this is soon intensively studied by many researchers in the world. Development of practically usable long wire materials is vigorously advanced—mainly, in Japan and USA. Today, it is reported in Japan that a 200 m-long electrical wire capable of obtaining a superconduction current or “supercurrent” as large as 200 A was manufactured in success.
While Y-based superconductor is in the way of reaching completion of the fabrication process thereof, its major applications considered are electrical coils to be used in large magnetic fields and wires to be used under relatively small magnetic fields created by Y-based superconductor. When a supercurrent flows in magnetic fields, it receives the so-called Lorentz force, resulting in electrical resistance taking place in the current. This leads to a decrease in critical current density Jc. As well known among those skilled in the art, magnetic fluxes that form a magnetic field cause superconduction properties to be impaired at every part if these magnetic fluxes are movable within supercurrent-obtainable regions. This would result in significant deterioration of superconductor wire characteristics to one-hundredth ( 1/100) or below as a whole.
It is the pinning center that suppresses such unwanted movement of magnetic fluxes leading to characteristics deterioration, thereby to improve the superconduction properties. By intentionally creating magnetic flux-passable non-superconductive regions, magnetic fluxes are prevented from moving into other superconductive portions, thus obtaining high properties of a superconducting wire in its entirety. In the case of foreign matter with no superconductivity being introduced as the pinning center, high superconductivity is practically obtainable in magnetic fields as a whole, although such portion becomes non-superconductor. For this reason, since the discovery of material with superconductivity, diligent and intensive studies have been continued to find the way of effectively introducing the pinning center into superconductor.
In recent years, several approaches are being vigorously studied to use PLD method to introduce, as the pinning center, a hetero-phase having a nano-size width, called the “nanorod.” Using BaZrO3 or BaSnO3 or else, an ultrafine linear nanorod is introduced into inside of a superconductor. Then, by causing magnetic fluxes to pass through this nanorod, let it function as the pinning center to thereby prevent degradation of superconduction properties occurring due to magnetic fluxes at other portions. It has been reported that in nanorod-introduced superconductors, a supercurrent obtained is enhanceable up to a fivefold to tenfold in the presence of a strong magnetic field of about 5 webers per square meter (weber/m2) or “teslas (T),” when compared to a superconductor with no nanorods introduced thereinto. A wire made of such nanorod-introduced superconductor is suitable for application to coils of the type creating a strong self magnetic field.
However, it is also known that when introducing a certain material with no supercurrent flowability, such as BaZrO3, into YBa2Cu3O7-x superconductor to thereby form a film, followed by execution of pure oxygen anneal for converting the superconductor from a tetragonal crystal structure into an orthorhombic crystal structure, the resulting superconduction properties are badly affected due to unintentional material diffusion into adjacent or nearby superconductors. According to recent reports, this results not only in a decrease in critical current density Jc but also in a decrease in superconductivity transition temperature Tc. Thus, the nanorod-added superconductor is considered to be rather limited in industrial applicability: it is merely suitable for strong magnetic field coexistence applications, such as coils for example. On the contrary, in those applications with no needs for large magnetic fields, it seems better in some cases to employ the pinning center introduction scheme that does not rely upon the use of nanorods, in which the degradation of superconduction properties hardly occurs.
An electric power cable used for long-distance large-power transmission is thought to be one example of the applications that do not require the use of nanorods. This power cable is typically installed in such a manner that both-way wires are basically designed to make a pair for canceling out a magnetic field, although this depends on the voltage to be sent. These wires are spaced apart from each other by a predefined distance for high voltage power transmission. In this case, each wire experiences application of a magnetic field which is created by the other. Although this magnetic field is not as large as that of a coil, this magnetic field can badly affect a superconductor, resulting in a decrease in allowable value of a supercurrent flowing in the superconductor.
In addition, the power transmission cable is under the need for cooling down a long and large system associated therewith; so, it is difficult to use refrigerators in order to cool the system successfully. Only one approach to reaching this goal is to employ a cooling technique using liquid nitrogen with its boiling point of 77.4K. As Y-based superconductors are merely 90.7K in superconductivity transition temperature Tc, a decrease in Tc occurring due to the nanorod introduction leads to an increase in risk of quenching, which is caused by local heat elevation in practical implementations.
For power cables, it is desirable to use magnetic field characteristics-improving techniques which are without introduction of nanorods. One of these techniques is to improve magnetic field characteristics by appropriate design of a low-oblique angle crystal grain boundary. The low-angle grain boundary means, in most cases, that grain planes of a superconductor are coupled together while forming therebetween a small plane-direction bond angle of about four (4) degrees or less. It is known that if the oblique angle is kept less than or equal to 4 degrees, the resulting superconductor is less in damageability. It is also known that the grain boundary thereof functions as the pinning center.
This low-angle grain boundary introducing technique is different depending on a superconductor film fabrication process used. In the PLD method or else, it often depends on a film forming temperature, oxygen partial pressure, to-be-formed plume position, etc. Therefore, when reduction to practice, it is almost impossible to precisely control an ultrafine structure while at the same time increasing reproducibility, although rough control is attainable as a whole. In PLD method, a plume is formed by irradiation of a laser beam onto a target; so, as the target is trenched more deeply, the plume can change in shape, resulting in likewise changes in material deposition rate and crystal orientation degree, causing the reproducibility to decrease accordingly.
Regarding the low-angle grain boundary formation, recent studies have revealed that the use of TFA-MOD method is effective for forming the intended grain boundary. While various types of growth mechanisms using this technique have been proposed until today, a crystal grain or particle is formed by creation and growth of the nuclear or “core” of a superconducting grain according to a quasi-liquid phase network model. An oblique angle to be formed by neighboring planes thereof is as small as 0.4 degrees in average so that a pinning center-introduced structure is obtained, which avoids deterioration of superconduction properties.
The density of crystal grains is controllable by adjustment of nucleation frequency and grain growth rate. These are controllable by macro parameters, such as a firing process temperature and partial pressures of oxygen and water vapor. Thus, a high-characteristics superconductor is readily obtainable with increased reproducibility, as suggested by U.S. Pat. Nos. 6,172,009 and 6,610,428, for example. The process of fabricating a superconductor film using TFA-MOD method has its ability to form low-angle grain boundaries and thus is thought to be a promising technique adaptable for the manufacture of superconductors to be used under certain environments with co-presence of relatively weak magnetic fields approximate to a self magnetic field, such as power cables for example.
Unfortunately, prior known low-angle grain boundary forming techniques using the TFA-MOD method are faced with a problem as to deterioration of superconduction properties. This can be said because a once-created low-angle grain boundary readily disappears with growth since the bond angle stays as small as 0.4 degrees in average. Thus it is predicted that the properties decrease with an increase in film thickness—in reality, some reports revealed this fact.
It has traditionally been known that when a film of Y-based superconductor is formed on or above a single-crystalline substrate made of LaAlO3, a great number of a/b-axis oriented crystal grains or particles with the c-axis (i.e., the <001> direction) being laterally laid down are formed, resulting in superconduction properties being degraded significantly. It has also been known that even for film fabrication on an intermediate layer made of CeO2 with the c-axis being rarely apt to lay down, deterioration of properties under self magnetic fields takes place with an increase in thickness of a growing film. These causes are considered to be deeply related to the above-stated low-angle grain boundary reduction. Accordingly, it has been desired to provide a new and improved approach to improving the superconduction properties.